The present invention relates to a high-speed voltage detector utilizing an electrooptic crystal.
FIG. 9 shows a conventional example of a high-speed voltage detector 1 called an E-O sampling apparatus. The high-speed voltage detector 1 includes a short-pulsed light source 2, a high-speed optical device 3 such as a high-speed photodetector, a light intensity modulator 4 including a polarizer 4A, optical modulator 4B and analyzer 4C, an optical delay device 5 and a photodetector 6. Short pulse light emitted from the short-pulsed light source 2 is split into two beams by a beam splitter 7. One beam is applied to high-speed optical device 3, whose output is provided to optical modulator 4B as an electrical signal to be measured. The other beam from beam splitter 7 is applied to the optical delay device 5 via a mirror 8A. Output light, which has been delayed by a predetermined time, is applied, via a mirror 8B, to light intensity modulator 4, where it is subjected to intensity modulation in accordance with the electrical signal (i.e., voltage) being applied to the optical modulator 4B and is passed through analyzer 4C to reach the photodetector 6.
As shown in FIG. 10, the voltage V being applied to optical modulator 4B and the intensity I of the light applied to the photodetector 6 via the analyzer 4C has a relationship as represented by following equation (1): EQU I=I.sup.0 sin .sup.2 {(.pi./2)(V/V.pi.) . . . (1)
where V.pi. is a half-wave voltage of the optical modulator 4B. Accordingly, photodetector 6 detects the light whose intensity has been modulated by the electrical signal to be measured according to equation (1).
An output signal of the photodetector 6 is amplified by an amplifier 9, and then provided to an X/Y recorder 10. The X/Y recorder 10 reproduces the whole waveform of the electrical signal under measurement using a delay time signal X from optical delay device 5 and a signal Y from the photodetector 6 while the delay time is continuously varied to change the sampling point. The principle of this measurement is described below in detail.
It is assumed that a waveform of the electrical signal to be measured and the input short pulse light to the optical modulator 4B has a relationship as shown in FIG. 11. At delay time Ta (caused by the optical delay device 5 and measured from the reference time point), point A of the electrical signal is measured. Since the voltage is zero at delay time Ta, no change occurs in the output signal of the photodetector 6.
At delay time Tb, point B is measured to generate an output that depends on a voltage Vb. Similarly, at delay time Tc, a voltage Vc at point C is measured. In this manner, the whole waveform of the electrical signal is measured.
The optical delay device 5 can operate slowly and, as a benefit of the sampling-type measurement, it suffices that the photodetector 6 just follow the variation of the delay time. Therefore, the photodetector 6 can also have a slow response speed.
Since the optical modulator 4B operates very fast, the response speed of the voltage detector 1 is determined by a width of the pulse light emitted from the short-pulsed light source 2.
However, as shown in FIG. 10, in the high-speed voltage detector 1, the voltage V applied to the optical modulator 4B and the intensity I of the output light from the analyzer 4C do not have a linear relationship. Therefore, in order to determine the voltage V from the light intensity I, a conversion reverse to the FIG. 10 characteristic needs to be performed. In doing so, however, in the range of small voltages V where the intensity I varies very little with respect to a variation of the voltage V, precise measurements can hardly be performed.
In order to solve this problem, usually a quarter-wave plate is disposed between the polarizer 4A and the analyzer 4C to provide an optical bias so as to establish an operating point at V =V.pi./2, and measurements are performed while applying a small modulation voltage v (&lt;&lt;V.pi./2) in the vicinity of the operating point. Since, in this case, an approximately linear relationship holds between the modulation voltage v and the modulated output light intensity I, a conversion reverse to the FIG. 10 characteristic is not required. However, this measurement method still has a problem that it only enables a measurement of a small voltage v (&lt;&lt;V.pi./2). That is, this type of voltage detector cannot be applied to measurement of such a large voltage V as is about the same as or larger than the half-wave voltage V.pi..
Further, in the high-speed voltage detector 1, when the voltage V is gradually increased to reach V.pi., the output light intensity I starts to decrease as understood from FIG. 10, which also prevents correct measurements.